Existing high performance heat sinks are characterized by a high fin density design, i.e., a fin population about twice that which can be normally produced in a standard production process. In this case, the surface area has a major influence on the overall heat transfer capability of the heat sink. Additionally for existing heat sinks having tightly spaced fins, the heat transfer coefficient is determined by the hydraulic diameter of the heat sink design. Hydraulic diameter is generally defined in the art as four times the area of the channel (i.e., space or distance between adjacent fins) divided by the perimeter of the channel. Thus, the smaller the hydraulic diameter, the higher both the heat transfer coefficient and the heat transfer of the heat sink.
One such design is illustrated in U.S. Pat. No. 4,777,560 by Herrell et al in which a high performance, high fin density heat sink is described. According to Herrell et al., various alternative heat sink construction techniques are described that produce high fin density design. An inherent disadvantage of the design is the inability to maximize the surface area of each individual fin. Approximately 25% to 33% of the potential individual fin surface area is not available, as this area is in contact with the adjacent fin (See for instance Herrell et al, FIGS. 1, 2, 3 and 4). In addition, heat sinks based on FIGS. 1 and 2 in Herrell et al have an internal plenum that further decreases available surface area for a given volume of a heat sink design. Thus, Herrell et al do not teach maximizing heat sink surface area, for a given heat sink volume.
In U.S. Pat. No. 5,304,846 to Azar et al, a heat sink design is disclosed that maximizes fin surface area in a high performance, high fin density heat sink. According to Azar et al., the manufacturing techniques disclosed are crystal-orientation-dependent etching, precision sawing, electric discharge machining, or numerically controlled machining. A major shortcoming of the Azar et. al. heat sink design is that they are generally difficult to manufacture. Additionally, the Azar et. al. heat sink requires enormously high production cycle time to manufacture which, of course, makes them cost ineffective.
In U.S. Pat. No. 4,884,331 by Hinshaw, a method of manufacturing a pin-finned heat sink from an extrusion is described. According to the cross cut machine method disclosed in Hinshaw, the maximum pin fin density that can be achieved is limited to what is obtainable by an extrusion process. This latter limitation clearly would not be acceptable in the heat sink design of the present invention. Another shortcoming of Hinshaw is that only square or rectangular pin fins can be manufactured, no round or elliptical profiles are available.
Moreover, there exists various heat sink manufacturers that offer bonded fin heat sink assemblies in which each fin in the assembly is individually bonded into a heat sink base. (See for instance, catalogue material on Augmented surface Bonded Heat Sinks published by AAVID.TM. Thermal Technologies, Inc. (March 1996). A major shortcoming, however of the AAVID.TM. heat sinks is there enormously high cost. This cost is related directly to the labor required to individually arrange each fin on some sort of support or substrate and high production cycle time.
Commonly owned U.S. patent applications Ser. Nos. 08/959,692, filed Oct. 29, 1997 and D.76,236, hereby incorporated herein by reference, disclose cost effective techniques, based upon dual extruded or dual cast heat sinks, to manufacture high fin density heat sinks. In brief, upper and lower heat sink elements described in these patent applications are combined together, with the lower fins being bonded into the upper recesses and the upper fins being bonded to the lower recesses. In these cases, due to the extrusion and die cast tooling limitations, the fin height to thickness ratio typically cannot exceed 10:1 for tightly packed fins. High fin density heat sink applications that require taller fins cannot be met with this dual extruded and bonded technique.
Therefore, a need persists for a high performance, high fin density, extruded, tiered heat sink with a high fin to thickness ratio, that maximizes heat sink surface area and is cost effective and simple to manufacture.